1. Field of the Invention
The present invention relates generally to fourdrinier paper machines. More specifically, the present invention relates to deckle structures for confining the papermaking stock pond carried on the fourdrinier screen.
2. Description of the Prior Art
Fourdrinier paper machines are characterized by a closed loop web formation screen driven over an open, flat table surface. Extremely dilute, aqueous papermaking stock is jetted upon the traveling screen from a horizontally elongated nozzle; usually associated with a stock accumulation chamber called a headbox.
As the traveling screen carries the stock flow from the slice jet landing zone, aqueous vehicle, i.e., water, drains through the screen to leave the fiber constituent of the papermaking stock accumulated upon the upper screen surface as a consolidated mat.
Between the stock landing zone and that longitudinally displaced point along the screen belt traveling route whereat the mat consolidates into a paper web, the stock is supported on the screen surface as a liquid pond of diminishing depth. Without lateral containment, lateral liquid stock flow cross-directionally sweeps fiber stock towards the screen sides thereby undesirably tapering the paper web edge thickness.
To prevent such undesirable thickness tapering along the paper web edges, lateral pond confinement structures called "deckle boards" are positioned above and along the screen edges in the machine direction from the slice landing zone. Traditionally, deckle boards are similar to a pair longitudinal dams, each extending along the screen traveling direction respective to each lateral edge of the screen with the screen per se running under the deckle boards.
A more recent innovation to the deckle structure has been to combine the deckle board with a screen edge cupping rail located outboard of the deckle board, as represented by U.S. Pat. No. 4,968,387 to R. L. Beran et al. The curled screen edges, traveling along respective, oppositely cupped rail profiles, hydraulically confine the stock pond. The deckle boards, internally of the cupped rails, are vertically positioned above the screen as to leave a substantial hydraulic channel beneath the lower deckle board edge. Machine white water fills the flow channel between the cupping rail and the outside surface of the deckle board. The inside faces of the deckle boards delineate the outer edge limits of the stock fiber. Standing waves generated in the stock pond are permitted to pass under the deckle board into white water channel and dissipate up the edge cup profile without reflection.
All deckle structure, whether of the traditional design or that using cupped rails, is positioned within close proximity of the energetically traveling stock pond. The structure is located within a virtual mist of fiber particles being continuously splashed from the traveling stock pond. These fiber particles have a high adhesive affinity for any solid surface such as is offered by the deckle structure. Fiber coatings continue to accumulate and soon begin to flake off in agglomerated chunks and fall into the fresh stock pond for web processing. Such web integrated chunks of agglomerated old fiber disrupt the web quality and runnability.
Although the prior art, as represented by U.S. Pat. No. 3,607,624 to W. R. Moody, has partially recognized the value of protecting the deckle structure with a continuously flowing water film, that recognition did not teach a functional structure that would adequately accomplish the objective. Many portions of the Moody structure are not water film flushed and are fiber accumulation surfaces.